It is standard practice to deform a semifinished sheet of glass while it is still very hot and somewhat plastic from its invariably substantially two-dimensional planar shape into a more complex three-dimensional shape. This procedure must be carried out while avoiding point stress which would mar the workpiece so that portions of it would be weakened and form optical distortions.
It is possible as suggested in German Pat. No. 1,010,245 to simply press a flat plastic sheet workpiece between two matching nonplanar dies, but such a procedure invariably mars the surfaces of the workpieces and therefore cannot be used for high-quality glass items. Hence suction is used to hold the workpiece on the forming die, as this style of holding is extremely gentle, the holding force being spread perfectly uniformly over the entire surface of the face of the sheet. Thus the surface of the workpiece will normally remain perfectly smooth, and the workpiece thickness will remain perfectly uniform, giving it good optical properties.
German patent document No. 3,109,149 describes a vacuum die that is used for this purpose. It has a face formed as a plurality of adjacent and outwardly open recesses or cells defining compartments opening at the face. These compartments can be individually evacuated. In addition the entire mold can be deformed from a start position in which the face of the mold is substantially planar to a finish position in which the face has the shape that is to be imparted to the workpiece.
Thus the die in the start position is applied normally down against the planar top face of the workpiece, which is held in a forming station on a conveyor, and the compartments are evacuated. This action pulls the glass sheet into tight contact with the still planar die face. Then the die is lifted to pick the workpiece up off the conveyor, and is deformed into the finish position, drawing the workpiece into the desired shape. The glass is given some time to harden in this finish position, then the vacuum is cut and the workpiece is freed from the die, which is then returned to its start position for another shaping operation.
Such dies can work on several workpieces at the same time, in which case they must normally be subdivided transversely as well as longitudinally into separate compartments. Thus the loss of suction at the gap between adjacent workpieces will not cause one of them to be dropped.
The main problem with such an apparatus and method is that the die is an extremely complex piece of equipment and is very hard to control accurately. It is in fact impossible to form certain shapes as the die cannot physically be deformed from the planar start position into some shapes. In addition in some arrangements the workpiece is stretched at the joints between adjacent relatively movable die portions, creating weakened regions that form optical distortions.